1. Field of the Invention
The present invention relates to fabrication of transistors in integrated circuit devices. Specifically, the present invention provides an improved method for fabricating an integrated circuit device including N-channel and P-channel transistors having selectively hardened gate oxides.
2. State of the Art
State of the art integrated circuit (IC) devices generally require transistors including hardened gate oxides. As feature dimensions continually decrease, gate oxides are hardened in order to prevent diffusion of dopants from overlying layers into and through the gate oxide, to prevent breakdown at voltages below normal operating parameters, and to confer resistance to hot electron degradation. State of the art transistors including hardened gate oxides, therefore, often possess performance and reliability advantages relative to transistors including non-hardened gate oxides applied in the same context. However, hardened gate oxides are not desirable in all IC applications, and known methods for hardening gate oxides often require significant design and performance tradeoffs.
While it is generally preferred to harden the gate oxides of P-channel devices due to the nature of P-type dopants, such is not the case for N-channel devices. Hardening of N-channel devices is generally not necessary as N-type dopants do not readily diffuse through non-hardened gate oxides. Moreover, hardening N-channel devices is often undesirable due to compounding performance problems. As is well known, hardening of gate oxides included in N-channel devices leads to significant threshold voltage (VT) roll-off. While VT roll-off can be counteracted through enhancement implants, the increased dopant concentration resulting from enhancement implants causes additional performance problems, such as refresh degradation and reduced surface mobility. Therefore, it would generally be advantageous not to harden the gate oxides of N-channel devices included within an IC device.
Despite the difficulties generally resulting from hardening the gate oxides of N-channel devices, however, the ability to selectively harden the gate oxides of N-channel devices in particular instances would be advantageous.
Additionally, as is also well known, it is often desirable to include P-channel or N-channel devices having gate oxides of varying thicknesses within a single IC device. For instance, it is beneficial to provide N-channel devices with gate oxides which are thicker than the thin, hardened gate oxides generally included in P-channel devices. Increasing the thickness of non-hardened N-channel gate oxides increases the VT of the corresponding N-channel devices, thereby decreasing control and performance the same. Moreover, it may also be advantageous to fabricate an IC device including hardened P-channel or N-channel devices incorporating hardened gate oxides of varying thicknesses.
Therefore, an ideal method for fabricating IC devices would facilitate simple and low cost fabrication of IC devices including P-channel and N-channel devices which have been selectively hardened and which may include hardened or non-hardened gate oxides of varying thicknesses. However, known methods for hardening gate oxides generally do not provide the flexibility needed to enable selective hardening of gate oxides within an IC device. Furthermore, though it is possible to fabricate hardened gate oxides of varying thickness using known techniques, such techniques generally require additional etch steps, which are costly and serve as an additional source of error in a fabrication process.
For example, known methods for hardening gate oxides included in an IC device, such as dynamic random access memory (DRAM) device, often require blanket hardening of a gate oxide layer deposited over a semiconductor substrate. During subsequent fabrication steps, both N-channel and P-channel gate oxides must then be formed using the blanket hardened gate oxide layer. Consequently, every one of the N-channel and P-Channel devices included in the subsequently formed IC device includes a hardened gate oxide. Blanket hardening processes simply do not enable selective hardening of particular areas of the gate oxide layer and, thereby, compromise the quality of IC devices fabricated by such methods.
Additionally, blanket hardening techniques cause difficultly in fabricating IC devices including gate oxides of varying thicknesses. Hardened oxide layers generally will not grow significantly during subsequent thermal oxidation processes. Therefore, to fabricate an IC device having gate oxides of various thicknesses using a blanket hardening process, the gate oxide layer must be formed such that, after hardening, the hardened gate oxide layer is as thick as the thickest desired gate oxide. The hardened gate oxide layer must then be selectively etched back to a desired thickness where P-channel or N-channel devices having thinner gate oxides are to be formed. Such a process is disadvantageous because it adds the cost and complication associated with one or more additional etch steps. Moreover, known etching processes are difficult to control where only minute amounts of material must be removed. Thus, as the thickness of state of the art gate oxides shrinks well below 70 xc3x85, the need to etch back a hardened gate oxide layer becomes increasingly problematic and can only serve as a source of error, decreasing fabrication throughput as well as device reliability.
As can appreciated, an improved method for fabricating IC devices including selectively hardening gate oxides is needed. Such a method should not only enable fabrication of P-channel and N-channel devices including selectively hardened gate oxides, but the improved method should also enable fabrication of such devices including hardened or non-hardened gate oxides of varying thicknesses without requiring additional etch steps.
The present invention provides a method for fabricating IC devices which answers the foregoing needs. The method of the present invention includes providing a semiconductor substrate having a gate oxide layer formed there over. A resist is then formed over the gate oxide layer and patterned to expose one or more areas of the gate oxide layer which are to be hardened. The exposed portions of the gate oxide layer are then hardened using a true remote plasma nitridation (RPN) scheme or a high density plasma (HDP) RPN scheme. Because the RPN scheme used in the method of the present invention runs at low temperature, the patterned resist remains stable through the RPN process. Therefore, those areas of gate oxide layer which are exposed by the patterned resist are selectively hardened by the RPN treatment, while those areas covered by the patterned resist remain unaffected.
The method of the present invention may also include additional steps for growing or hardening non-hardened areas of the gate oxide layer remaining after the first RPN treatment. For example, the non-hardened portion of the gate oxide layer, or a portion thereof, may be grown to provide thick, non-hardened gate oxides for N-channel devices. Or alternatively, the non-hardened portion of the gate oxide layer, or a portion thereof, may be grown and subsequently hardened, to provide P-channel devices or N-channel devices having hardened gate oxides of varying thickness. The portion of the gate oxide layer hardened by the first RPN need not be masked during subsequent thermal oxidation and hardening steps because, once hardened, the gate oxide will not grow significantly when exposed to subsequent thermal oxidation processes. In fact where implants are required through the gate oxide layer, it is beneficial not to mask the hardened portion of the gate oxide layer during subsequent thermal oxidation, as such a process will heal any implant damage done to the gate oxide layer.
As can be easily appreciated by one of skill in the art, the present invention enables fabrication of an IC device including N-channel and P-channel devices having hardened or non-hardened gate oxides of varying thicknesses without the need to the etch steps made necessary by known methods. Moreover, the method of the present invention may be easily integrated into known fabrication processes using known technology, and, because the method of the present invention does not require etching of the gate oxide layer to achieve gate oxides of varying thickness, the method of the present invention is cost effective and will continue to be useful as device dimensions shrink beyond the dimensions of those devices currently considered to be state of the art.